As I understand it the vet is legally obliged to provide this and not allowed to charge as such, BUT they are allowed to pass on the admin 'costs' of the prescription, which you could argue to be £6.
The Competition Commission introduced these rules as a three-year trial in 2005. It may all change come this October - ie, they may be allowed to charge again. I'm not sure if a decision has been made about this yet, not quite up to speed although I should be!
The aim of this, I think, was to get vets to charge properly for their professional time (ie, consultation prices go up) and rely less on profit from medicines, by making the medicines a free market.
My dog is also on Metacam for osteoarthritis and gets through about four of those 100ml bottles a year. I have such a good relationship with my vet, however, (sometimes consultations that should be £50 slip through uncharged, because he has looked after my animals since I was little and will quite often waive fees as a goodwill gesture) that I would rather any profit from the meds went to him, so continue getting the Metacam from him.
If he was one of the corporates that are springing up (large companies taking over) then I probably wouldn't feel this way and would say sod it and get the prescription...
Not sure something ongoing like like would be insured after the first year, depends how good your policy is though.